Oxides of nitrogen in the form of Nitrogen Oxide (i.e., NO) and Nitrogen Dioxide (NO2) (oxides of nitrogen can generally be referred to as: NOx) are generated by the burning of fossil fuels. Along with NOx from vehicles, NOx from fossil fuel fired industrial and commercial heating equipment (e.g., furnaces, ovens, etc.) is a major contributor to poor air quality and smog.
Flue gas recycling is an industry accepted way to achieve low NOx emissions in fossil fuel fired combustion applications. Numerous field and laboratory studies have proven the beneficial effect of recycling flue gas using a variety of fossil fuel burner-sealed fired chamber test arrangements. However, the addition of flue gas recycling to any fired application requires increased equipment complexity, capital, and/or operational expense.
One method to achieve flue gas recycling using premixed burners (using a combustion air and fuel gas mixture), is to have the flue gas ducted back to a point near the combustion air intake where it can enter the combustion air fan to be mixed with the combustion air and fuel gas. This method requires additional piping and apparatus around the burner and boiler (or other sealed fired chamber).
It also requires an enlargement or upsizing of the combustion air fan to handle the increased volume of the added flue gas. Larger fans have increased cost and use more electricity per unit of heat produced. Further, these fans can become fouled due to the hot, corrosive flue gas and require the use of higher cost alloy materials, and/or additional cleaning and maintenance to keep the fan operational.
Another method, applicable to non-premixed burners, is to use an auxiliary fan to suction flue gas from the exhaust stack or fired chamber, and discharge that flue gas into the burner housing where it mixes with the incoming combustion air provided by the combustion air fan. This method requires additional flue gas piping and an additional corrosion resistant, high temperature rated fan to transport the hot flue gas.